Question 6.SSE.26: An alkene (A) C16H16 on ozonolysis gives only one product (B......
An alkene (A) C_{16}H_{16} on ozonolysis gives only one product (B) C_8H_8O . Compound (B) on reaction with NaOH/I_2 yields sodium benzoate. Compound (B) reacts with KOH/NH_2NH_2 yielding a hydrocarbon (C) C_8H_{10} . Write the structures of compounds (B) and (C). Based on this information two isomeric structures can be proposed for alkene (A). Write their structures and identify the isomer which on catalytic hydrogenation ( H_2/Pd-C ) gives a racemic mixture.
[IIT, 2001]
Alkene (A) is symmetric because it gives a carbonyl compound (C8H8O) on ozonolysis.
C_8H_8O \overset{NaOH + I _2}{\xrightarrow {\hspace{2 cm}} } C_6H_5COONaCompound (B)
As, it gives iodoform test with a mixture of NaOH and I_2 ,so it must have CH _3 CO – group. Thus the possible structure of compound (B) is C _6 H _5- CO – CH _3 \text {. }
\begin{matrix}C_6H_5COCH_3 & + & 4 NaOH & + & 3 I _2 & \longrightarrow \\ CHI _3 \downarrow & + & C _6 H _5 COONa &+& 3 H _2 O&+& 3 NaI \\ \text {Iodoform} \end{matrix} \begin{matrix} C_6H_5 \\ & \diagdown \\ && C & = & O & + &NH_2 \bullet NH_2 \\ & \diagup \\ CH_3 \\&&& \underset {\text {Compound (B) Hydrazine}}{} &&& \underset {\text {Hydrazine}}{}\end{matrix} \begin{matrix} KHO & C _6 H _5- CH _2- CH _3\\ {\xrightarrow {\hspace{2 cm}} }\\ \text {Wolff-Kishner reduction} & \left( C _8 H _{10}\right)( C ) \\ \left(- N _2,- H _2 O \right) \end{matrix}According to the structure of compound (B), the compound (A) is
\begin{matrix} C _6 H _5& -& C& =& C &-& C _6 H _5 \\ && | &&| \\ && CH_3 && CH_3 \\ &&& \text {cis}\end{matrix}or
\begin{matrix} && CH_3 \\ && | \\ C _6 H _5& -& C &=& C &-& C _6 H _5 \\ &&&& | \\ &&&& CH_3 \\ &&& \text {trans}\end{matrix}So
\begin{matrix} C_6H_5 &&&&&& C_6H_5 && C_6H_5 &&&&&& CH_3 \\ & \diagdown &&&& \diagup &&&& \diagdown &&&& \diagup \\ && C&=& C &&& \overset{\text{Ozonolysis}}{\underset{2 C _6 H _5 COCH _3}{\xrightarrow {\hspace{2 cm}} }} & or && C &=& C \\ &\diagup &&&& \diagdown &&&& \diagup &&&&\diagdown \\ CH_3 &&&&&& CH_3 && CH_3 &&&&&& C_6H_5\end{matrix}
Symmetric alkenes show the properties of geometrical isomerism. So its cis and trans isomers are possible. In these isomers trans isomer gives racemic mixture on hydrogenation (H_2/Pd -C) .
\begin{matrix} C_6H_5 &&&&&& CH_3\\ & \diagdown &&&&\diagup \\ && C& =& C &&&& \overset{H _2 / Pd – C}{\xrightarrow {\hspace{2 cm}} }\\ & \diagup &&&& \diagdown \\ CH_3 &&&&&& C_6H_5 \end{matrix} \begin{matrix} && C_6H_5 &&&&&&&C_6H_5\\ && | &&&&&&&|\\ CH_3 & – & C & – & H &&&H&-&C& – & CH_3 \\ && | &&&&&&&|\\ H & – & C & – &CH_3 &&& CH_3 & – & C & -& H\\ && | &&&&&&&|\\ && C_6H_5 &&&&&&& C_6H_5\\ &&&&& \text {Racemic mixture}\end{matrix}Hence, compounds
\begin{matrix}&&& C_6H_5 &&&&&& CH_3\\ &&&& \diagdown &&&& \diagup \\ &&&&&C & =& C \\ &&&&\diagup &&&&\diagdown \\ &\text{(A)}&=&CH_3 &&&&&&C_6H_5 \end{matrix}
or \begin{matrix}&&& C_6H_5 &&&&&& C_6H_5\\ &&&& \diagdown &&&& \diagup \\ &&&&&C & =& C \\ &&&&\diagup &&&&\diagdown \\ &&&CH_3 &&&&&&CH_3 \\\\\\\\\\ \end{matrix}
\text { (B) }= C _6 H _5 COCH _3\text { (C) }= C _6 H _5- CH _2- CH _3